Secondary batteries, such as alkaline secondary batteries, equal in size and output voltage to dry batteries (primary batteries prescribed by IEC60086 (JISC8500)) and interchangeable with the dry batteries have been spreading widely due to the recent growing trend toward global environmental protection. Examples of known charging techniques for secondary batteries include a technique which uses a contactless power transmission technique. Examples of known conventional techniques for charging secondary batteries interchangeable with dry batteries using the contactless power transmission technique include an electromagnetic induction method (see, for example, Patent Literatures 1 to 5).
However, the contactless power transmission by electromagnetic induction method is capable of transmitting electric power over a very short distance and is liable to suffer a significant drop in power transmission efficiency if there is even a slight shift in positional relationship between a power transmitter-side coil and power receiver-side coil. This makes it necessary to establish the positional relationship accurately, presenting a problem in terms of convenience. Also, the contactless power transmission by electromagnetic induction method has a problem in terms of safety because any metal object lying on a power transmission path will be heated by induction heating.
Consequently, contactless power transmission techniques based on a magnetic field resonance method have been attracting attention in recent years (see, for example, Patent Literatures 6 to 10). The magnetic field resonance method, which is entirely different from the electromagnetic induction method, uses magnetic field resonance whereby oscillation of a magnetic field generated when an electric current flows through a power transmitter-side coil is transmitted to a receiver-side resonance circuit oscillating at the same frequency. Contactless power transmission by magnetic field resonance method is capable of transmitting electric power over a longer distance than the electromagnetic induction method, and is almost free of decrease in power transmission efficiency even if there is some shift in positional relationship between the power transmitter-side coil and power receiver-side coil, and is thus quite advantageous in terms of convenience. Also, the contactless power transmission by magnetic field resonance method uses a smaller magnetic field than the magnetic induction method and allows only resonance circuits of a specific resonance frequency to receive power, and thus develops little induction heat. Also, objects to be charged can be selected according to resonance frequency. Furthermore, whereas the power transmitter side and power receiver side are in a one-to-one relationship in the case of the magnetic induction method, the magnetic field resonance method allows a single power transmission coil to transmit electric power to plural power receiver coils. Thus, the magnetic field resonance method is quite advantageous in terms of convenience in this respect as well.